pipe cross fitting water supply shut off valve for toilet

Hi, I'm planning to install cross fitting at the outlet of shut off valve for water supply line going to toilet tank and need to know what size should I use? The idea is to use one of the cross outlets for toilet tank as usual, the second cross outlet for bidet sprayer and the third outlet to go to a mini water heater to get some warm water for sprayer.

I'm not sure what you mean by a "cross fitting" but the output side of the typical shutoff valve for a toilet is a compression fitting (usually 3/8"). That is the biggest challenge because beyond supply lines there are not a lot of plumbing fittings that mate to the male side of compression. There are shut off valves that have two outputs, typically used under the kitchen sink to supply the faucet and dishwasher (3/8" x 3/8") or faucet and ice maker (3/8" x 1/4"). But you need one more output.

There's a couple of ways you could approach the problem. One would be to replace the existing shutoff valve, add a series of two Tee fittings, one after the other, and come off of those with individual shutoff valves for the toilet, bidet, and water heater. Keeney makes a fitting what they call "add a tee" which would allow you to add on to your existing shutoff valve (you'd need two).

I don't know off hand is Home Depot sells Keeney (Lowe's does) or if anyone else makes a similar fitting.

If you do decide to replace the existing shutoff valve and you have something that looks like this:

A pretty chrome tube that comes of the wall (or floor) and smoothly transitions into the shutoff valve, do not cut off the shutoff valve thinking you can attach something else. This kind of fitting slides over ordinary 1/2" copper pipe can only be unsweated (unsoldered) because it has an outside diameter that no other plumbing fitting mates to. If you don't feel comfortable doing that, then you need to find a plumber or use another method.

The easiest way to add outlets would be to use two of these fittings stacked on top of your stop valve. They are sold at Home Depot stores and are typically located on a shelf just below the rack of flexible supply lines in the plumbing department. Link